Most air conditioners in use today reject heat through air-cooled condensers. These units include the typical residential split-system air conditioner with the condensing unit outdoors and packaged rooftop units that are often used on commercial buildings.
The cold refrigerant within the air conditioning unit absorbs heat from indoors through the evaporator. The refrigerant evaporates or boils, capturing a great deal of energy as it absorbs heat and cools the airflow within the home or building. The warm refrigerant vapor is then returned to the compressor where the pressure is raised significantly, causing a commensurate increase in refrigerant temperature. The high pressure, hot vapor is then passed to the condenser coils. The refrigerant vapor temperature is typically 30 F higher than the ambient or outdoor temperature. Outdoor air is forced over the coils to reject or remove the heat that was absorbed back in the evaporator coil plus the energy that was added through compression. In the heat rejection process, the vapor condenses into a high pressure liquid refrigerant. The warm liquid refrigerant is then carried to an expansion valve near the evaporator that rapidly drops the refrigerant pressure with a commensurate drop in temperature. The refrigerant then enters the evaporator, and the cycle repeats.
The energy that is rejected or removed from the condenser is equal to the energy that is absorbed as heat in the evaporator plus the energy that is added as work by the compressor in raising the pressure of the refrigerant. Or put another way, the cooling capacity of the evaporator, and indeed the amount of compressor work needed is directly related to the heat or energy that can be rejected through the condenser coil. It is therefore beneficial that the heat transfer through the condenser be maintained at the highest possible level. The heat transfer rate from the condenser coil is modeled by the well-known convection rate equation:Q=h*A*(Trefg−Tair)  (1)    Where: Q=heat transfer rate [Btu/hr or Watts]            h=convection heat transfer coefficient [Btu/(hr*ft2*F) or W/(m2*C)]        A=coil surface area [ft2 or m2]        Trefg=refrigerant temperature [F or C]        Tair=air temperature [F or C]        
The optimal air conditioning performance is achieved by maintaining a high heat transfer rate, or heat rejection, Q, through the condenser.
One way to increase Q is to increase Trefg. This can only be achieved by increasing the compressor output pressure. However, the compressor work or energy input must be increased to increase head pressure and Trefg, which ultimately costs more work energy than cooling gained. The air conditioner control system does increase pressure in response to higher outdoor temperatures (Tair) and, as described below, in response to blockage of the coil or decreased convection coefficient.
There are two practical approaches for maximizing the air conditioner's efficiency by maintaining the highest possible heat rejection, Q. First, one can insure that the effective coil area, A, and the nominal convection coefficient, h, are not reduced or compromised. It is also possible to increase h by increasing the air velocity across the coil. This is only practical by changing the condenser fan(s) or increasing its speed. Increasing the speed will increase the brake horsepower of the fan motor. Second, one can reduce the air temperature, Tair, entering the condenser coil.
Both approaches, maintaining A and h and decreasing Tair are addressed by the present invention and are discussed below.